Conventionally, Cu—Fe—P alloy containing Fe and P are generally used as a copper alloy for semiconductor lead frames. Examples of these Cu—Fe—P alloys include, for example, a copper alloy (C19210 alloy) containing Fe: 0.05 to 0.15% and P: 0.025 to 0.040%; and a copper alloy (CDA194 alloy) containing Fe: 2.1 to 2.6%, P: 0.015 to 0.15%, and Zn: 0.05 to 0.20%. When an intermetallic compound such as Fe or Fe—P is precipitated in a copper matrix, these Cu—Fe—P alloys exhibit high strength, high electric conductivity, and high thermal conductivity among copper alloys, and therefore these alloys have been used as the international standard alloys.
The recent advancement of the large-capacity, miniaturization, and high-performance of semiconductor devices used in electronic apparatuses has urged the growing reduction in the cross-sectional area of lead frames adopted in the semiconductor devices; thereby, there is a demand for higher strength, higher electric conductivity, and higher thermal conductivity. With the demand, there is also a demand for higher strength and higher thermal conductivity to a copper alloy sheet used in lead frames for the semiconductor devices.
On the other hand, plastic packages of semiconductor devices have an advantage that the package in which a semiconductor chip is sealed by a thermosetting resin is excellent in economical efficiency and mass productivity, and therefore they become mainstream. These packages are increasingly thinner with the recent demand for miniaturization of electronic parts.
In assembling the packages, a semiconductor chip is heated to be adhered to a lead frame by using an Ag paste, etc., or soldered or brazed with Ag via a plated layer made of Au and Ag or the like. Thereafter, the package is generally encapsulated with a resin and subsequently connected to an outer lead by electroplating.
The most serious challenge with respect to the reliability of these packages is a package crack or peeling occurring upon implementation. Peeling of a package occurs due to a thermal stress generated in the subsequent heat treatment, when the adhesion between the resin and the die pad (portion where a semiconductor chip of a lead frame is mounted) is deteriorated after assembling the semiconductor package.
On the other hand, a package crack occurs through the following processes: a mold resin absorbs moisture from the air after assembling a semiconductor package, and the moisture vaporizes during a heating process in the subsequent implementation; and when a crack is present inside the package at the time, the moisture is applied to the peeling plane, which acts as an internal pressure; and a swelling is then caused in the package by the inner pressure, or a crack is caused when the resin is weak against the inner pressure. When a crack is caused in a package after the implementation, moistures and impurities are incursive therein to cause the chip to be corroded, impairing the function as a semiconductor. In addition, the swelling of a package results in poor appearance and lost of its commodity value. In recent years, such problems involving package cracks and peeling have been remarkable with the above advancement of thinning of the packages.
The problems involving package cracks and peeling are caused by the deteriorated adhesion between the resin and the die pad. An oxidation film of a lead frame base material has the greatest influence on the adhesion between the resin and the die pad. The lead frame base material has been subjected to various heating processes for producing sheets or lead frames. Accordingly, an oxidation film with a thickness of tens to hundreds of nanometers is formed on the surface of the base material before a plating process is performed with Ag or the like. On the surface of the die pad, a copper alloy and the resin are in contact with each other via the oxidation film, and hence the peeling of the oxidation film from the lead frame base material directly leads to the peeling between the resin and the die pad, causing adhesion between the resin and the lead frame base material to be remarkably deteriorated.
Accordingly, the problem involving the package crack and the peeling depends on the adhesion between the oxidation film and the lead frame base material. Therefore, the above Cu—Fe—P alloy sheet with high strength is required as a lead frame base material to have a high adhesion property with the oxidation film formed on its surface through various heating processes.
In addition, heating temperatures in the above various heating processes for producing copper alloy sheets and lead frames, are increasingly higher for the purposes of improving productivity and efficiency. For example, in the lead frame production process, a heat treatment after a press process, etc., is required to be conducted at a higher temperature and in a shorter time. With such a heating temperature being higher, a new problem arises that the oxidation film formed on a lead frame base material tends to peel off from the material more easily due to roughness and fineness of the film, as compared to a previous oxidation film that is formed by heating at a lower temperature.
Techniques for improving resistance of peel off of oxidation film have been conventionally proposed, although the number of the proposals is small. For example, it is proposed that crystalline orientation in the surface layer of a copper alloy is controlled in Patent Document 1. That is, Patent Document 1 proposes that, in crystalline orientation in the surface of a copper alloy base material for lead frames, which is evaluated by the thin film method using an XRD, resistance of peel off of oxidation film can be improved by a ratio of the peak intensity of {100} to the peak intensity of {111} being 0.04 or less. It is noted that Patent Document 1 includes every kind of copper alloy base materials for lead frames; however, Cu—Fe—P alloys substantially exemplified are only Cu—Fe—P alloys with an Fe content of 2.4% or more, which is a large content.
Taking the surface roughness of a Cu—Fe—P alloy sheet into consideration, Patent Documents 2 and 3 propose that resistance of peel off of oxidation film of the sheet can be improved by making a centerline average roughness Ra 0.2 μm or less and a maximum height Rmax 1.5 μm or less, in measurements of the surface roughness. More specifically, in Patent Documents 2 and 3, the surface roughness is controlled by the type (surface roughness) of a rolling roll in the cold-rolling.
Also in recent years, with increasing applications of Cu—Fe—P alloys and the advancement of the lightweight, thinning, and miniaturization of electric and electronic apparatuses, these copper alloys are also required to have higher strength, higher electric conductivity, and excellent bendability. As for such bendability, the copper alloys are required to endure sharp bending such as U-bending or 90° bending after notching.
On the other hand, it is conventionally known that bendability can be improved to some extent by grain refining or by controlling the dispersion state of dispersoids/precipitates (see Patent Documents 4 and 5).
In Cu—Fe—P alloys, it is also proposed that the microstructure thereof is controlled in order to improve properties such as bendability. More specifically, it is proposed that: a ratio, I(200)/I(220), of the intensity, I(200), of x-ray diffraction of (200) to the intensity, I(220), of x-ray diffraction of (220) is 0.5 or more and 10 or less; or orientation density: D (Cube orientation) of Cube orientation is 1 or more and 50 or less; or a ratio: D (Cube orientation)/D(S orientation) of the orientation density of Cube orientation to the orientation density of S orientation, is 0.1 or more and 5 or less (see Patent Document 6).
It is also proposed that a ratio, [I(200)+I(311)/I(220)], of a total of the intensity, I(200), of x-ray diffraction of (200) and the intensity, I(311), of x-ray diffraction of (311) to the intensity, I(220), of x-ray diffraction of (220) is 0.4 or more (see Patent Document 7).
On the other hand, the copper alloy sheets provided with high strength are also required to have workability so as to be formed into the lead frames with reduced cross-sectional areas. Specifically, a copper alloy sheets are subjected to a stamping process so as to be formed into lead frames, and hence the copper alloy sheets are required to have excellent stampability. The demand also exists in the applications in which the copper alloy sheets are stamped, other than the application of lead frames.
Conventionally, in order to improve the stampability of Cu—Fe—P alloy sheets, techniques for controlling chemical components in which trace additives such as Pb and Ca are added or a compound that is a starting point of a break is dispersed, or techniques in which a grain size, etc., is controlled, have been widely used.
However, these techniques have problems that the controls per se are difficult to be carried out, these controls adversely affect other properties, and therefore a production cost is increased.
On the other hand, it is proposed that the stampability and the bendability of a Cu—Fe—P alloy sheet are improved taking the structure thereof into consideration. For example, Patent Document 8 discloses a Cu—Fe—P alloy sheet containing Fe: 0.005 to 0.5 wt %, P: 0.005 to 0.2 wt %, and further either or both of Zn: 0.01 to 10 wt % and/or Sn: 0.01 to 5 wt % if needed, with the remainder of Cu and inevitable impurities. In Patent Document 8, the stampability is improved by controlling an integration degree of crystal orientations of the copper alloy sheet (see Patent Document 8).
More specifically, in Patent Document 8, the integration degree is controlled with the use of the fact that: as the copper alloy sheet is recrystallized and a grain size of the structure becomes larger, an integration ratio of {200} plane and {311} plane on the sheet surface is larger; and when the copper alloy sheet is rolled, an integration ratio of {220} plane is larger. Characteristically, Patent Document 8 is intended to improve the stampability by increasing an integration ratio of {220} plane on the sheet surface relative to {220} plane and {311} plane. More specifically, assuming that, on the sheet surface, an intensity of x-ray diffraction of {200} plane is I[200], that of {311} plane is I[311], and that of {220} plane is I[220], [I[200]+I[311]]/I[220]<0.4 should be satisfied.
The afore-mentioned Patent Documents 6 and 7 also disclose copper alloy sheets of which stampability is improved. (see Patent Documents 6 and 7).
Patent Document 9 proposes that I(200)/I(110) should be 1.5 or less in order to improve the flexibility of a Cu—Fe—P alloy sheet (see Patent Document 9).
In addition, it is known that, in order to improve the bendability of a Cu—Ni—S alloy (Corson alloy), a ratio of the uniform elongation to the total elongation, which are among the tensile properties of the copper alloy, is made 0.5 or more, although the copper alloy belongs to another copper alloy system (see Patent Document 10).
A copper alloy sheet provided with such high strength is subjected to a stamping process and a bending process or the like, followed by being plated with Ag, etc., and is then formed into lead frames.
However, there sometimes occurs unusual precipitation of the plating partially (locally) on the surface of the Ag plating or the like, the unusual precipitation being observed by a microscope as a projection of the plated layer, like a dot illustrated by the arrow in FIG. 3 (SEM picture substituting for a drawing, magnification 500). When such unusual precipitation of the plating occurs, the lead frame is no longer used as a semiconductor lead frame because a bonding defect is induced.
The unusual precipitation of the plating does not occur on the whole surface of the plating, nor in a large amount in every semiconductor lead frame to be produced. However, for a highly-efficient mass production line of semiconductor lead frames, when the unusual precipitation of the plating occurs in semiconductor lead frames to be produced, even if the number of the occurrences is very small, i.e., in ppm order, there is inevitably a serious influence on the production speed and the production efficiency of the line.
At present, the unusual precipitation of the plating is presumed to be caused by the residue of the coarse inclusions (oxides and dispersoids) that are formed in the casting and melting process onto the surfaces of a final product, or by the surface defects such as coarse pore, which are formed due to hydrogen. It is because, on the surface of a final product immediately beneath the plating layer where the unusual precipitation of the plating occurs, coarse inclusions (oxides and dispersoids) or surface defects such as coarse pores formed due to hydrogen, are mostly present and remain.
It is inevitable that a Cu—Fe—P alloy contains hydrogen and oxygen to some extent during the casing and melting process, and coarse inclusions (oxides and dispersoids) formed in the casting and melting process remain up to a final product sheet and pores formed due to hydrogen appear as surface defects.
Many techniques in which a copper alloy for semiconductor lead frames is provided with high strength and high formability such as stampability and bendability, have conventionally been proposed. However, techniques in which the platability of a copper alloy for semiconductor lead frames, in particular, the platability of a Cu—Fe—P alloy is improved, and more particularly, the afore-mentioned unusual precipitation of the plating is suppressed, have not been proposed so many.
Among them, a technique in which the platability of a copper alloy sheet is improved by containing Fe: 1.5 to 2.3 wt % or P: 0.015 to 0.045 wt %, which are relatively large amounts, is proposed (Patent Document 11). In Patent Document 11, it is also proposed that intercrystalline cracks are prevented by containing C: 10 to 100 ppm, which is also a relatively large amount.    [Patent Document 1] Japanese Patent Laid-Open No. 2001-244400    [Patent Document 2] Japanese Patent Laid-Open No. H2-122035    [Patent Document 3] Japanese Patent Laid-Open No. H2-145734    [Patent Document 4] Japanese Patent Laid-Open No. H6-235035    [Patent Document 5] Japanese Patent Laid-Open No. 2001-279347    [Patent Document 6] Japanese Patent Laid-Open No. 2002-339028    [Patent Document 7] Japanese Patent Laid-Open No. 2000-328157    [Patent Document 8] Japanese Patent Laid-Open No. 2000-328158    [Patent Document 9] Japanese Patent Laid-Open No. 2006-63431    [Patent Document 10] Japanese Patent Laid-Open No. 2002-266042    [Patent Document 11] Japanese Patent No. JP 2962139